The present invention relates to multiplexing of information displays and, more particularly, to novel methods for matrix addressing cholesteric-to-nematic phase change liquid crystal displays.
It is known that matrix addressing may be utilized with liquid crystal displays utilizing a liquid crystal material employing the cholesteric-to-nematic phase change. Hitherto known matrix addressing methods have typically maximized the RMS change of the voltage in each display cell formed at an intersection between the perpendicularly-arranged row and column electrodes placed upon opposite sides of the display liquid crystal layer. By merely maximizing the RMS voltage change, relatively poor performance is achieved, as the response time is typically too long to allow a high order of multiplexing, while unsatisfactorily small contrast ratios are achieved.
It has been suggested to matrix address a cholesteric liquid crystal matrix by sequentially addressing the electrode intersections thereof with waveforms which have, for a period of time, a zero value of voltage at each intersection. The zero intersection voltage is utilized to erase the intersection display state, in a time slot immediately prior to a time slot in which the desired display state is "written" into the display intersection. Thus, when a scan pulse sequentially traverses a plurality of scan electrodes, the liquid crystal material is first caused to go into the focal-conic state and is then realigned into the homeotropic nematic state where desired. This erasing of an intersection previously in the homeotropic nematic state, and subsequent realignment into the same homeotropic nematic state, causes a noticeable blink in the display, which blink occurs at the multiplex rate. During the time interval between each "write" time interval and the next "erase" time interval, the particular display intersection is maintained in a desired state by placing a holding voltage (typically having a value greater than the value required to turn the intersection cell "off" and less than a value required to turn that intersection's cell to the "on" condition. The intersection data waveform is typically utilized as the holding voltage waveform, whereby the amplitude of the data waveform must be a compromise between havng the desired homeotropic state of an intersection cell decay before the cell can be refreshed (if the holding voltage is of too small a value) and having the intersection cell partially activated out of the focal-conic state (if too large a holding voltage is used).
It is therefore highly desirable to provide a method of matrix addressing a multiplexed cholesteric liquid crystal display, wherein the matrix addressing method only erases those intersection cells which have been, or which are going to be, placed in the light-scattering state (or the light-absorbing state if a dichroic dye guest is present). Those intersection cells of the matrix display which have been, or are going to be, in the light-transmitting homeotropic state are to have the homeotropy thereof enforced; blinking is thereby eliminated and no compromise of holding voltage magnitude need be made.